1. Field of the Invention
This invention relates to an auto-focus camera.
2. Related Background Art
FIG. 4 of the accompanying drawings shows part of an auto-focus camera according to the prior art. When a half-depression switch, not shown, is closed, an oscillator 10 starts operating and electric power is supplied to a driver 20, and an infrared light-emitting diode (hereinafter simply referred to as the light-emitting diode) 30 emits light in synchronism with the oscillator 10. Also, upon closing of the half-depression switch, a photo-taking lens is axially moved and correspondingly thereto, the direction of irradiation of the light-emitting diode 30 changes. In other words, the emitted light from the light-emitting diode 30 scans the object field correspondingly to the amount of axial movement of the photo-taking lens. The emitted light from the light-emitting diode 30 is reflected by the object field and enters two-division silicon photodiodes (hereinafter simply referred to as the photodiodes) D1 and D2 constituting a light-receiving element 40. Output signals i.sub.1 and i.sub.2 photoelectrically converted by the photodiodes D1 and D2 are amplified by head amplifiers 50 and 51 and input to a differential amplifier 60, whereby a difference signal v.sub.3 is obtained. The difference signal v.sub.3 is detected in synchronism with the oscillator 10 by a detector 70 and converted into a DC level voltage V.sub.o. The output V.sub.o of the detector 70 is input to a comparator 80 and compared with a reference voltage V.sub.r. When the detection output V.sub.o becomes smaller than the reference voltage V.sub.r, the output V.sub.out of the comparator 80 is inverted and a photo-taking lens driving motor 100 is stopped through a motor driver 90. That is, the design is such that the photo-taking lens is axially moved toward the in-focus position until the outputs i.sub.1 and i.sub.2 of the two-division photodiodes D1 and D2 become equal to each other. Thus, at this point of time, the emitted light is in a predetermined area in the object field.
However, in the above-described auto-focus camera according to the prior art, when the difference signal v.sub.3 which is the output of the differential amplifier 60 is detected to obtain a DC level detection output, the detection output varies at a predetermined time constant, and the scanning speed of the light-emitting diode 30 relative to the object field is constant. If the time constant is great, the variation in the detection output is slow and thus responsiveness is poor. Conversely, if the time constant is small, S/N ratio becomes bad, and this has led to a problem that distance measurement accuracy is low.